The present invention relates to sonar system useful for active submarine detection especially in shallow water littoral environments.
A system according to the invention uses a broadband source, such as a multiple pulse airgun array, whereby the broad transmitted bandwidth of, say, from below 50 to above 600 Hz can be processed simultaneously in a multiplicity of narrower sub-bands, to seek the maximum target return against propagation loss, noise and reverberation at any aspect the target chooses to present (or inadvertenty presents) to the detection system (hydrophone array). The system may also use a broadband single transmission for use with broadband processing techniques to select a narrow band having maximum target return.
It is well known that the target strength of a submarine varies with submarine class, frequency and aspect. Furthermore, the propagation loss through the water varies with frequency, depth, and bottom composition. Additionally, reverberation caused by the surface and bottom structure can be a severe impediment to active submarine detection.
In light of the above many factors that can influence submarine detectability, there has arisen a school of thought that has focused on using broadband single-shot transmissions to blanket the frequency ranges that propagate well, to minimize reverberation through reduced xe2x80x9cspotxe2x80x9d size, and to use neural networks or other advanced processing means to recognize submarine target echoes from those of rocks and irregular bottom features. These steps have met with some success, but the reduction of reverberation and clutter, particularly, has been sufficiently limited in certain instances to make it desirable to seek other (or additional) solutions. Reverberation encompasses acoustic reflections from the irregular structure of the water column due to temperature, salinity and current fluctuations. Clutter generally encompasses returns from irregular structure in the bottom including fluctuations in density, irregular rock formations, etc.
Reverberation and clutter have been addressed by transmitting a sequence of closely-spaced transmissions and matched-filter processing the returned signals. Such processing can provide 20 to 40 dB of reverberation/clutter suppression and contribute to the improved illumination of a moving target.
Although matched-filter processing can be done on a broad band basis, there is considerable advantage to restricting processing bandwidth to narrower (e.g., 50 Hz) bands which provide steeper slopes to the processed Q functions, allowing for the detection of slow moving targets, such as {fraction (1/2 )} to 1 knot observed velocity with respect to the bottom.
For any broadband source (for example, an airgun array), a large number of multiple, narrow (e.g., 50 Hz) frequency bands are available for use, and, in principle, specific bands could be selected at any given time and location by the operator, based upon prior knowledge of the propagation characteristics of the particular region involved and/or the particular nature of the target being pursued. This approach, which depends on operator selection, while probably effective in some instances, may not be optimum. In general, it is unlikely that the operator can make the choice of the optimum sub-band in real time.
If the operator had a clue as to the aspect the target is presenting to him he could use information on target strength as a function of aspect, that can come from measurements and/or modeling of a scale model submarine of the design that is being pursued, to make a selection of xe2x80x9cbestxe2x80x9d frequency band or bands.
However, the propagation loss between target and interrogating sonar can also fluctuate with frequency, sometimes exhibiting nulls. This fluctuation may defeat or compromise any effort to choose a frequency band based upon the suspected target aspect alone.
Of the many features that impact on target detectability, a feature that is generally under the control of the sonar operator is the frequency or frequency bands of the active transmission. In the case of a wide-band transmission (e.g., from the airgun source) usefully extending from, say, 50 Hz to 600 Hz, the question is, xe2x80x9cWhich sub-band or bands should be employed by the operator?xe2x80x9d
The invention continually takes advantage of the full bandwidth capability of the airgun source and automates much of the submarine search process, to reduce decision making on the part of the operator and yet provide the operator with more information than has heretofore been available.
The invention provides a sonar system with novel signal processing of returns from a target which is interrogated by a broadband transmission. A multiplicity of contiguous narrow (e.g., 50 Hz) bands are match-filter processed in parallel to seek the frequency band giving, on a joint basis, the best contribution of target response and least propagation loss between target and interrogator.
Accordingly, a system provided by the invention is operative to examine, on each beam, all of the relatively narrow (e.g., 50 Hz) frequency bands simultaneously and to instruct the processor to select and exhibit the joint optimum responses for that beam. Each 50 Hz band between 50 Hz and 600 Hz (or other suitable overall band) can be examined in parallel for each beam. The question of which 50 Hz band or bands best present a target response is accommodated since all bands are being searched simultaneously and a computer-aided detection processor with appropriate training can make a detection decision based upon the best band response.